The solution lies in the power point economy. The campaign needs to have enough encounters per day that the mythic players would run out of power points unless they conserve them.
Don't restrict the use of power points. Let the players choose how they want to spend them. A fun game rewards players for smart play. A DM that nerfs smart play soon has a boring game. Instead, provide more challenges, and more difficult challenges; enough that amazing initiative is no longer a "no-brainer" because the potential depletion of power points is always on their minds.